Regular Expressions in C

Muhammad Zeeshan Oct 12, 2023

C C Regex

Regular expressions in C
Construct a Regular Expression Using the regcomp() Method in C
Compare Strings to a Compiled Regular Expression Using the regexec() Method in C

Regular expressions in C will be the topic of discussion in this article, along with an example.

Regular expressions in C

Regular expressions, consisting of a string of characters, may be used to discover search patterns. String matching and other similar applications make extensive use of it.

They are a standardized approach to matching pattern sequences with character sequences. C, C++, Java, and many other major programming languages use it.

After learning about regular expressions, let’s look at some POSIX-specific library patterns.

Finding the letters or numbers enclosed by brackets is easy using [].
[:number:] can be used to locate any number.
[:word:] can search for letters, digits, and underscores.
[:lower:] is used to locate the alphabet’s lowercase letters.

Construct a Regular Expression Using the `regcomp()` Method in C

The regular expression regcomp() method is used for compiling or constructing regular expressions. It requires the regex, expression, and flag.

The expression is a string type, regex is a reference to a memory region where the expression is matched and saved, and the flag is used to identify the kind of compilation.

It will return the value 0 if the compilation has been completed without error. And it will output Error code if the expression cannot be compiled successfully.

Syntax:

regcomp(&regex, expression, flag)

Example:

Create a variable of the regex type, and give it a name like we did r1.
```
regex_t r1;
```
Now, you need to build a variable that will hold the result of the regex generation process, as you can see below:
```
int regval;
```
Generate a regex by calling the function:
```
regval = regcomp(&r1, "[:word:]", 0);
```
Regular Expression compiled successfully. will be shown after the compilation process.
```
if (value == 0) {
  printf("Regular Expression compiled successfully.");
}
```
Else, it will throw an exception with the message An Error Occurred..
```
else {
  printf("An Error Occurred.");
}
```

Complete Source Code:

#include <regex.h>
#include <stdio.h>

int main() {
  regex_t r1;
  int regval;
  regval = regcomp(&r1, "[:word:]", 0);
  if (regval == 0) {
    printf("Regular Expression compiled successfully.");
  } else {
    printf("An Error Occurred.");
  }
  return 0;
}

Output:

Regular Expression compiled successfully.

Compare Strings to a Compiled Regular Expression Using the `regexec()` Method in C

It is possible to compare a string to a pattern using the regexec() function. It considers some parameters: the first is a pattern that has been precompiled, a string that contains the pattern to be looked for, information about the location of matches, and flags that describe a change in the behavior of matches.

Syntax:

regexec(&regex, expression, 0, NULL, 0);

If there is a match, this will return the value 0. And if there is no match, it will return REG NOMATCH.

Example:

To begin, we will create a function named DisplayPattern which will display the result. If the value is 0, it will show Pattern found; if it is not equal to 0, it will simply show Pattern not found.; else, it will show an error prompt with the message An error occurred..
```
void DisplayPattern(int val) {
  if (val == 0) {
    printf("Pattern found.\n");
  } else if (val == REG_NOMATCH) {
    printf("Pattern not found.\n");
  } else {
    printf("An error occurred.\n");
  }
}
```
Now, we’ll create the Main function where we’ll create a regex_t type variable named r1.
```
regex_t r1;
```
Create three variables for the return type named data1,data2,data3.
```
int data1;
int data2;
int data3;
```

Then, create a regEx and compare the pattern with the string in `reg`.

data1 = regcomp(&r1, " I am a writing c language", 0);
data1 = regexec(&r1, "Hi there its me Zeeshan", 0, NULL, 0);

Add a few more regEx as shown below, and compare the pattern with the string in `reg`.

data2 = regcomp(&r1, "I am a writing c language", 0);
data2 = regexec(&r1, " Lets do coding", 0, NULL, 0);

data3 = regcomp(&r1, "My Name is Zeeshan Khan", 0);
data3 = regexec(&r1, " My Name is Zeeshan Khan", 0, NULL, 0);

At last, display patterns using the DisplayPattern function that we created before.
```
DisplayPattern(data1);
DisplayPattern(data2);
DisplayPattern(data3);
```

Complete Source Code:

#include <regex.h>
#include <stdio.h>

void DisplayPattern(int val) {
  if (val == 0) {
    printf("Pattern found.\n");
  } else if (val == REG_NOMATCH) {
    printf("Pattern not found.\n");
  } else {
    printf("An error occurred.\n");
  }
}

int main() {
  regex_t r1;
  int data1;
  int data2;
  int data3;
  data1 = regcomp(&r1, " I am a writing c language", 0);
  data1 = regexec(&r1, "Hi there its me Zeeshan", 0, NULL, 0);

  data2 = regcomp(&r1, "I am a writing c language", 0);
  data2 = regexec(&r1, " Lets do coding", 0, NULL, 0);

  data3 = regcomp(&r1, "My Name is Zeeshan Khan", 0);
  data3 = regexec(&r1, " My Name is Zeeshan Khan", 0, NULL, 0);

  DisplayPattern(data1);
  DisplayPattern(data2);
  DisplayPattern(data3);
  return 0;
}

Output:

Pattern not found.
Pattern not found.
Pattern found.

Author: Muhammad Zeeshan

I have been working as a Flutter app developer for a year now. Firebase and SQLite have been crucial in the development of my android apps. I have experience with C#, Windows Form Based C#, C, Java, PHP on WampServer, and HTML/CSS on MYSQL, and I have authored articles on their theory and issue solving. I'm a senior in an undergraduate program for a bachelor's degree in Information Technology.

Regular Expressions in C

Regular expressions in C

Construct a Regular Expression Using the `regcomp()` Method in C

Create a variable of the regex type, and give it a name like we did `r1`.

Now, you need to build a variable that will hold the result of the regex generation process, as you can see below:

Generate a regex by calling the function:

`Regular Expression compiled successfully.` will be shown after the compilation process.

Else, it will throw an exception with the message `An Error Occurred.`.

Compare Strings to a Compiled Regular Expression Using the `regexec()` Method in C

To begin, we will create a function named `DisplayPattern` which will display the result. If the value is 0, it will show `Pattern found`; if it is not equal to 0, it will simply show `Pattern not found.`; else, it will show an error prompt with the message `An error occurred.`.

Now, we’ll create the `Main` function where we’ll create a `regex_t` type variable named `r1`.

Create three variables for the return type named `data1`,`data2`,`data3`.

Then, create a regEx and compare the pattern with the string in `reg`.

Add a few more regEx as shown below, and compare the pattern with the string in `reg`.

At last, display patterns using the `DisplayPattern` function that we created before.

Regular expressions in C

Construct a Regular Expression Using the regcomp() Method in C

Create a variable of the regex type, and give it a name like we did r1.

Now, you need to build a variable that will hold the result of the regex generation process, as you can see below:

Generate a regex by calling the function:

Regular Expression compiled successfully. will be shown after the compilation process.

Else, it will throw an exception with the message An Error Occurred..

Compare Strings to a Compiled Regular Expression Using the regexec() Method in C

To begin, we will create a function named DisplayPattern which will display the result. If the value is 0, it will show Pattern found; if it is not equal to 0, it will simply show Pattern not found.; else, it will show an error prompt with the message An error occurred..

Now, we’ll create the Main function where we’ll create a regex_t type variable named r1.

Create three variables for the return type named data1,data2,data3.

Then, create a regEx and compare the pattern with the string in reg.

Add a few more regEx as shown below, and compare the pattern with the string in reg.

At last, display patterns using the DisplayPattern function that we created before.

Construct a Regular Expression Using the `regcomp()` Method in C

Create a variable of the regex type, and give it a name like we did `r1`.

`Regular Expression compiled successfully.` will be shown after the compilation process.

Else, it will throw an exception with the message `An Error Occurred.`.

Compare Strings to a Compiled Regular Expression Using the `regexec()` Method in C

To begin, we will create a function named `DisplayPattern` which will display the result. If the value is 0, it will show `Pattern found`; if it is not equal to 0, it will simply show `Pattern not found.`; else, it will show an error prompt with the message `An error occurred.`.

Now, we’ll create the `Main` function where we’ll create a `regex_t` type variable named `r1`.

Create three variables for the return type named `data1`,`data2`,`data3`.

Then, create a regEx and compare the pattern with the string in `reg`.

Add a few more regEx as shown below, and compare the pattern with the string in `reg`.

At last, display patterns using the `DisplayPattern` function that we created before.